Rats have long been a major threat to crops.

But not where farmers are using a hybrid type of corn developed by a subsidiary of American chemical giant Du Pont, Xinhua news agency cited local farmers as saying.

Liu Min, a farmer in Zhangqing Township of the northern Shanxi Province's Jinzhong City, hasn't seen big rats for the last three or four years - and it actually has him worried as he thinks about the possible cause.

Other farmers said big rats are only seen in barns storing traditional types of corn.

Meantime, abnormal things have happened to livestock fed by the new type of corn, according to the villagers.

Fewer local farmers are running pig farms now because sows are frequently experiencing immature deliveries and miscarriages.

Ewes are having the same problems, farmers said. And pigs, sheep and dogs are experiencing kidney and liver difficulties.

The area's farmers began planting the new type of corn, Pioneer 335 by Pioneer Hi-Bred International Inc, in 2004.

Producing better yields and containing more protein than regular strains, and promoted by countless neighborhood salesmen, the corn has been adopted by many farmers.

It is being planted this year in as many as 3 million hectares nationwide - 10 percent of the country's corn planting, Xinhua said.

Shanxi is the traditional base of China's corn industry, but many local strains of corn are no longer available. Local farmers are giving them up.

Pioneer Inc has denied that the strain developed in China in 2000 was genetically modified. But one of the corn's parents was a genetically modified type, according to its patent published online by US authorities.

"If it is true, the abnormal behaviors on animals are with reasonable explanation," an insider said, "because experiments around the world have discovered damages to kidney or liver or problems with reproduction or immunity of animals fed by genetically modified food."